Achieving ultra pure argon without evacuation has been demonstrated several times at Fermilab by using the argon piston, where argon is slowly fed into the bottom of a tank originally filled with air. The argon then pushes the air out the top of the tank, and molecular diffusion occurs at the interface of the two fluids during the purge. The upward velocity must be great enough to overcome this molecular diffusion, but slow enough that it remains laminar and no mixing occurs, roughly 1 m/hr has shown be successful in previous gas purging experiments for this gas pair. This document acts to explain the physics of a buoyancy assisted gas purge, and its applicability for achieving very pure gases without vessel evacuation. It further analyzes the applicability of the gas purge in the MicroBooNE cryostat, a complex shaped, varying cross section cryostat with internal obstacles. Computational fluid dynamics code is used to numerically approximate the Navier-Stokes and mass diffusion equations and show the transient results for the gas composition of the cryostat space.